A poor fit will negate any physiologic evaluation of an implantable total artificial heart (TAH). Fifteen Holstein calves (weight, 65-111 kg) were used to measure intrathoracic dimensions for TAH design, to find correlations between intrathoracic and external parameters for preoperative animal selection, and to suggest minor modifications for fitting a TAH in calf chests. The following dimensions were found to be critical to avoid compromising venous return: 1) mitral annulus center (MVC)-sternum, 10.4 +/- 1.0 cm; 2) tricuspid annulus center (TVC)-sternum, 9.8 +/- 1.6 cm; 3) MVC-TVC, 5.5 +/- 0.4 cm; 4) MVC-diaphragm, 3.4 +/- 0.8 cm; 5) MVC-left chest wall, 4.5 +/- 0.9 cm; and 6) TVC-right chest wall, 4.5 +/- 1.0 cm. These dimensions had no statistically significant correlation with body weight, but three of them did with the following external parameters: 1) bronchial carina-sternum on chest radiograph, p less than 0.05; 2) dorsoventral dimension at xiphoid, p less than 0.05; and 3) circumference at the caudal end of the scapula, p less than 0.05. Based on these results, the port orientation of the left housing of this electrohydraulic TAH designed for humans was modified for in vivo evaluation in calves. No other pump features required change. These data should prove useful to others involved in TAH development.